Fluoropolymer Coated Membranes

Abstract

The present invention discloses fluoropolymer coated membranes and methods for making and using these membranes. The fluoropolymer coated membranes described in the current invention are prepared by coating a porous asymmetric membrane layer with a thin layer of fluoropolymer coating. The porous asymmetric membrane layer comprises an asymmetric cellulosic membrane, an asymmetric polymer membrane, or an asymmetric molecular sieve / polymer mixed matrix membrane with a low selectivity and high permeance. The fluoropolymer coating improves the selectivity of the porous asymmetric membrane layer and maintains the membrane performance with time. The fluoropolymer coated membranes are suitable for a variety of liquid, gas, and vapor separations such as desalination of water by reverse osmosis, non-aqueous liquid separation such as deep desulfurization of gasoline and diesel fuels, ethanol / water separations, pervaporation dehydration of aqueous / organic mixtures, CO2 / CH4, CO2 / N2, H2 / CH4, O2 / N2, olefin / paraffin, iso / normal paraffins separations, and other light gas mixture separations.

Description

BACKGROUND OF THE INVENTION[0001]Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Polymeric membranes have proven to operate successfully in industrial gas separations such as in the separation of nitrogen from air and the separation of carbon dioxide from natural gas. Cellulose acetate (CA) is a polymer currently being used in commercial gas separation. For example, UOP LLC's Separex™ CA membrane is used extensively for carbon dioxide removal from natural gas. Nevertheless, while they have experienced commercial success, CA membranes still need improvement in a number of properties including selectivity, performance durability, chemical stability, resistance to hydrocarbon contaminants, resistance to solvent swelling, and resistance to CO2 plasticization. Natural gas often contains substantial amounts of heavy hydrocarbons and water, either as an entrained liquid, or in vapor form, which may...

AI Technical Summary

Benefits of technology

[0009]The fluoropolymer coated membrane described in the current invention comprises a porous asymmetric membrane layer which is directly coated with a thin layer of fluoropolymer coating that provides improved selectivity and stable performance over a wider range of temperature, in the presence of high concentration of CO2 and / or in the presence of hydrocarbon contaminants. The porous asymmetric membrane layer with a low selectivity and high flux can be made from materials including cellulosic membranes, membranes formed from other polymers such as polysulfone, polyethersulfone, polyamide, polyimide, polyetherimide, cellulose nitrate, polyurethane, polycarbonate, polystyrene, polybenzoxazole, etc, as well as membranes formed from molecular sieve / polymer mixed matrix materials such as AIPO-14 / polyimide mixed matrix material and AIPO-14 / (polyimide+polyethersulfone) mixed matrix material. The fluoropolymer coating improves the selectivity of the porous asymmetric membrane layer and exhibits essentially no loss in selectivity or no loss in flux rates over a typical operating period. The term “essentially no loss in flux rates” means that the flux declines less than about 30%, and more particularly the flux rate declines less than 20% over a typical operating period.

Problems solved by technology

Natural gas often contains substantial amounts of heavy hydrocarbons and water, either as an entrained liquid, or in vapor form, which may lead to condensation within membrane modules.

The presence of more than modest levels of hydrogen sulfide, especially in conjunction with water and heavy hydrocarbons, is also potentially damaging.

Another issue of CA polymer membranes that still needs to be addressed for their use in gas separations is the plasticization of the polymer by condensable gases such as carbon dioxide and propylene that leads to swelling of the membrane as well as a significant increase in the permeability of all components in the feed and a decrease in the selectivity of CA membranes.

The challenge of treating gas, such as natural gas, that contains relatively large amounts of CO2, such as more than about 10%, is particularly difficult.

However, permeability and selectivity are both temperature-dependent.

There are however several other obstacles to use of a particular polymer to achieve a particular separation under any sort of large scale or commercial conditions.

One such obstacle is permeation rate or flux.

Such membranes have a serious shortcoming in that, in operation, the permeation rate and / or selectivity is reduced to unacceptable levels over time.

One reason for the decrease of permeation rate has been attributed to a collapse of some of the pores near the skinned surface of the membrane, resulting in an undue densification of the surface skin.

While TFC membranes are less susceptible to flux decline than phase inversion-type membranes, fabrication of TFC membranes that are free from leaks is difficult, and fabrication requires multiple steps and so is generally more complex and costly.

Plasticization occurs when one or more of the components of the mixture act as a solvent in the polymer often causing it to swell and lose its membrane properties.

It has been found that polymers such as cellulose acetate and polyimides which have particularly good separation factors for separation of mixtures comprising carbon dioxide and methane are prone to plasticization over time thus resulting in decreasing performance of these membranes.

The coating of such coated membranes comprising siloxane or silicone segments, however, is subject to swelling by solvents, poor performance durability, low resistance to hydrocarbon contaminants, and low resistance to plasticization by the sorbed penetrant molecules such as CO2 or C3H6.

In addition, gas separation membranes desirably have a high permeation rate to gases.